This invention relates to a process and apparatus for producing readily polymerizable vinylaromatic compounds. More particularly, this invention relates to a process and apparatus for inhibiting the formation of undesired polymeric material during distillative purification of vinylaromatic monomers.
Vinylaromatic monomers, such as styrene, alpha-alkylstyrene, vinyltoluene, divinylbenzene and the like, are important for their ability to form useful polymeric materials. These compounds are typically prepared by catalytic dehydrogenation of alkylaromatic compounds having corresponding carbon chains. The crude product of the dehydrogenation reaction, however, is a mixture of materials comprising in addition to the desired vinylaromatic monomer, various alkylaromatic compounds as well as oligomers of the desired monomer. These other substances must be separated from the vinylaromatic monomer to obtain a commercially useful product.
The useful method for separating a desired vinylaromatic monomer from the dehydrogenation product mixture is to pass the mixture through a distillation train in which lower boiling materials are first separated and then the desired monomer is distilled from the higher boiling materials. Such distillative separations are complicated by the fact that the tendency of the monomer to polymerize increases with increasing temperature. Thus, as the mixture is heated to distill it, the formation of undesired polymer increases and the yield of desired monomer decreases.
Various measures have been utilized to minimize the undesired polymer formation. Vacuum distillation has been resorted to to reduce the temperature to which the feed mixture must be heated. While this is helpful in reducing the formation of undesired polymeric material, substantial amounts of polymer still are formed.
Polymerization inhibitors have been added to the feed mixture. Known inhibitors further reduce the formation of undesired polymer, but still are not totally effective. Moreover, such inhibitors may be expensive and contribute substantially to the production costs for the vinylaromatic monomer.
A particular problem arises in areas in the distillation apparatus where there is little vapor motion, such as adjacent the undersides of the seal pans. In such low velocity areas, monomer vapors condense against the cool undersides of the pans and form droplets of liquid monomer which may polymerize and solidify before they grow large enough to drop down into the underlying tray. Masses of unwanted polymeric material thus build up in the distillation apparatus. Liquid phase active inhibitors do not prevent such deposits because the condensing vapors do not carry these inhibitors with them. Even the use of vapor phase active inhibitors is not totally effective in supressing the formation of such deposits, since the lack of vapor motion in the low velocity areas restricts the mixing of the inhibitor with the condensing vapors.
The continuing accumulations of undesired polymeric material thus require that a distillation apparatus used to purify vinylaromatic monomers be periodically shut down and cleaned of the fouling polymer. As the polymer is typically a dense hard material, considerable difficulty may be encountered in cleaning the distillation apparatus. The need for periodically cleaning contributes substantially to operating costs, and capital costs are also increased because additional distillation capacity must be constructed in order to compensate for the down time of the distillation apparatus.
Not only is the yield of monomer decreased and the productivity of equipment lessened by reducing effective capacity and by requiring periodic shutdowns for cleaning, but the insoluble residues produced by the undesired polymerization of the monomer constitute a waste material which must be disposed of. Governmental restrictions on waste disposal make this an ever increasing problem.
Despite the efforts of the prior art, there remains a substantial need for improved methods and apparatus for inhibiting the formation of undesired polymeric residues in distillation apparatus used to purify vinyl aromatic monomers.